Conventionally, there is a two-cycle engine or a four-cycle engine as an engine mounted on a vehicle such as a two-wheeled motorcycle and each of these engines is provided with an intake device. Some intake devices include fuel supply systems for supplying fuel to intake passages.
A mixture of air and fuel is supplied to such an intake device. Some intake devices include straightening plates disposed in intake passages in order to improve filling efficiency of the air-fuel mixture. However, straightening of the air-fuel mixture by using the straightening plate is not sufficient to atomize the air-fuel mixture and reduce harmful components in exhaust gas to ideal degrees.